As shown in FIG. 10, in a solid polymer fuel cell, an assembly (MEA: Membrane Electrode Assembly) comprising an electrolyte membrane 52 formed from a solid polymer film sandwiched between two electrodes, namely a fuel electrode 50 and an air electrode 54, is itself sandwiched between two separators 40 to generate a cell that functions as the smallest unit, and a plurality of these unit cells are then usually stacked to form a fuel cell stack (FC stack), enabling a high voltage to be obtained.
The mechanism for electric power generation by a solid polymer fuel cell generally involves the supply of a fuel gas such as a hydrogen-containing gas to the fuel electrode (the anode side electrode) 50, and supply of an oxidizing gas such as a gas comprising mainly oxygen (O2) or air to the air electrode (the cathode side electrode) 54. The hydrogen-containing gas is supplied to the fuel electrode 50 through fine passages that have been machined in the surface of the separators 40, and the action of the electrode catalyst causes the hydrogen to dissociate into electrons and hydrogen ions (H+). The electrons flow through an external circuit from the fuel electrode 50 to the air electrode 54, thereby generating an electrical current. Meanwhile, the hydrogen ions (H+) pass through the electrolyte membrane 52 to the air electrode 54, and bond with oxygen and the electrons that have passed through the external circuit, thereby generating reaction water (H2O).
Moreover, the two separators 20 that sandwich the MEA described above perform a role as partitions for separating the hydrogen gas and the oxygen gas, and also have a function of electrically connecting the stacked cells in a series arrangement. Furthermore, fine corrugated passages are formed in the surfaces of the two separators, and these passages function as gas distribution passages for distributing the hydrogen-containing gas and the oxygen-containing gas or air.
One example of the structure of a conventional cell is illustrated in FIG. 11 and FIG. 12. The cross-section along the line A-A′ of FIG. 12 is illustrated in FIG. 11.
As illustrated in FIG. 11 and FIG. 12, supply communication holes 12a, 12b and 12c through which the fuel gas, the oxidizing gas and cooling water are supplied, and discharge communication holes 14a, 14b and 14c through which the fuel gas, the oxidizing gas and the cooling water are discharged are provided at the respective ends of the two separators 110 and 120. Moreover, gas passages 152 and 154 that distribute the fuel gas and the oxidizing gas supplied from the supply communication holes 12a and 12b respectively are also provided in the separators 110 and 120 respectively. Furthermore, concave portions 106 and 116 are provided in the opposing surfaces of the separators 110 and 120 respectively, and sealing materials 60a and 60b that isolate the fuel gas and the oxidizing gas are provided on both surfaces at the edges of the assembly (MEA) 30. These sealing materials 60a and 60b are bonded to the two separators 110 and 120 via adhesive materials 70a and 70b respectively, thus completing formation of a cell.
However, in those cases where stainless steel (so-called SUS) is used for the separators, then as illustrated in FIG. 6, a passivation film 22 composed of a chromium oxide film is formed on the surface of an SUS separator substrate 20. On the other hand, in recent years there has been a trend towards using more environmentally friendly materials for the aforementioned adhesives and sealing materials. For example, there is a trend towards replacing conventional solvent-soluble lipophilic resins with highly hydrophilic aqueous resins. However, the passivation film 22 described above exhibits poor affinity for these types of aqueous resins. Accordingly, when the above aqueous resins are bonded directly to the SUS separator substrate 20, either as an adhesive or as a sealing material that requires no adhesive, the bonding strength is poor, meaning that when a plurality of fuel cells each having an aforementioned assembly sandwiched between a pair of separators are stacked together and pressure is applied via the manifolds to effect stack fastening, shearing stress is generated which can cause peeling of the resin. Furthermore, thermal expansion or the like generated during use of the fuel cell stack can also cause peeling of the resin, and in some cases there is a possibility that resin detachment may occur.
Furthermore, as illustrated in FIG. 13, in those cases where electrodeposition coating is used to form an aqueous resin on a SUS separator substrate 20 having a passivation film 22 composed of a chromium oxide film formed on the surface thereof, then as shown in FIG. 6, the affinity between the passivation film 22 and the resulting aqueous resin layer 26 is poor. As a result, when the SUS separator substrate 20 is dipped in the bath of the electrodeposition coating material, air tends to be incorporated, and the electrodeposition coating occurs with residual air bubbles 23 trapped at the surface of the SUS separator substrate 20, as shown in FIG. 7. As a result, a multitude of pinholes 27 having diameters from 50 to 100 μm are generated within the formed aqueous resin layer 26.
Accordingly, a method has been proposed in which an iron-based hydrated oxide film having a high affinity for both the passivation film formed on the surface of the SUS separator substrate and the aqueous resin layer is provided between the passivation film and the aqueous resin layer, and this iron-based hydrated oxide film enables the passivation film and the aqueous resin layer to be bound tightly together, forming a fuel cell separator having superior adhesion between the SUS separator substrate and the aqueous resin layer (for example, see JP 2007-242576 A).
Furthermore, another electrodeposition coating method has been proposed in which an anti-foaming agent is adhered to the surface of the object to be coated, prior to the electrodeposition coating, thereby reducing the incorporation of air during dipping of the object to be coated into the electrodeposition coating bath and suppressing the adhesion of air bubbles to the surface of the object to be coated within the bath, and as a result, localized non-adhesion of the electrodeposition coating material is prevented, and the occurrence of film defects such as pinholes in the electrodeposition coating film can be inhibited (for example, see JP 2007-84877 A).
Furthermore, JP 2004-59985 A discloses a wafer plating method in which during a plating treatment of the surface of a wafer that functions as a substrate, the wafer is first wetted with either water or a mixture of water and a surfactant, before being dipped in a plating liquid to effect the plating treatment.